1. Field of the Invention
This invention relates to a magnetic field-generating device which is used in medical-purpose nuclear magnetic resonance-computerized tomography (hereinafter referred to as NMR-CT) and which employs a permanent magnet.
2. Description of the Prior Art
In order to obtain a desired tomographic image of the human body by NMR-CT, a magnetic field of 1-10 KG (0.1-1 T) in strength and 10.sup.-4 or less in uniformity are required.
Recently, a magnetic field-generating device employing a permanent magnet has been considered as the most economical device for NMR-CT because it does not consume electric power and does not employ expensive coolant (such as helium), as do other types of magnetic field-generating devices employing normal-conducting or superconducting magnets.
Prior to the present invention, the present applicant has proposed several practical configurations of magnetic field generating devices which include permanent magnets therein (see U.S. Pat. No. 4,672,346 and EP 0161782 Al). One such device is shown in FIG. 9. It includes a pair of permanent magnet segments 1, 1 which are magnetized in the same direction and are spaced from each other so as to define an air gap 4 therebetween, their poles of different polarities being mutually opposed. The opposing surface of each permanent magnet segment has a pole piece 2 attached thereto which is formed with only an annular projection 5 or with both an annular projection 5 and a convex projection 6 located in the central portion of the annular projection, and these permanent magnet segments are magnetically coupled together by a yoke 3 to create a magnetic field within the air gap 4 (FIG. 9 shows the pole piece formed with both the annular projection and the convex projection).
It is known that the permanent magnet of a magnetic field-generating device can be made of a ferrite magnet, an alunico magnet, and a rare earth cobalt magnet. In this connection, the present applicant has proposed (in Japanese Patent Publication No. 61-34242) that the device can be remarkably miniaturized by using Fe-B-R series permanent magnet (B, Fe are the principal components; R is selected out of the light rare earth element group which is one of the rich resources and whose center is Nd or Pr) which provides a high energy product of more than 30 MGOe.
According to the foregoing prior technique, a pair of permanent magnet segments is normally made only by one of the aforementioned permanent magnets. For example, if the ferrite magnet only is employed, the device can be produced at a low cost, but is limited in the degree of miniaturization and the extent of field strength. On the other hand, if the rare earth magnet, such as an Fe-B-R series magnet is employed, this kind of magnet is effective to miniaturize the device and enhance the field strength, but is costly.
Nowadays, due to improvements in peripheral apparatus other than the magnetic field-generating device, it is possible to obtain a clear image even when the field strength within the air gap is of the order of 1-1.5 KG. Therefore, a novel magnetic field-generating device is demanded which is superior in practicability, i.e., is compatible with recent advances and economically advantageous, and especially, an effective disposition and structure of the permanent magnet serving as the magnetic field generating source is required.